Text subtitle display is a technique for displaying subtitles represented by text codes, in synchronization with pictures in a movie. This is one of the techniques unique to BD-ROM (Blu-ray Disc Read Only Memory) playback devices.
BD-ROM playback devices expand text codes constituting subtitles into bitmap with use of a font, and display the bitmap in synchronization with pictures in a video stream.
The following describes a conventional subtitle display technique. When figures and characters are displayed by a display apparatus such as a household television monitor, display data for each pixel is stored in a frame memory. Here, if each pixel holds color information such as α, R, G, and B values, the size of the frame memory increases. Suppose that each pixel holds color information indicating α, R, G, and B values, where each value is one byte. In this case, a 4-byte memory is required for each pixel. To avoid this situation, a color lookup table is used to reduce the size of the frame memory. The color lookup table shows, for each pixel, a correspondence between (i) an index value to be stored in the frame memory and (ii) color information. Since the color lookup table shows a correspondence between index values and color information, each pixel only holds an index value. The index values are converted into color information to display images, with reference to the color lookup table. In this way, each pixel in the frame memory only holds an index value. Therefore, in a case where the number of colors simultaneously displayable on a screen is 256 (i.e., 28), a 1-byte index value is assigned to one pixel, allowing a memory necessary for one pixel to be only one byte. In this way, using the color lookup table enables a reduction in the size of the frame memory of the playback device.
Such a technique for assigning the display colors of characters is disclosed, for example, in Patent Literature 1.